During the study, researchers found four genetic markers that may play a role in all five diseases. These markers were more common in people with the one of the psychiatric disorders compared to healthy people.

Two of these markers are found in genes thought to be involved in balancing the level of calcium in brain cells, the researchers said. (Calcium plays a role in many different cell functions, including communication between cells.) Of these two markers, one has been previously linked to bipolar disorder and depression, and the other to schizophrenia.

Although these five psychiatric disorders are thought of as separate conditions, they share some symptoms. For instance, mood and thinking problems can occur with schizophrenia, bipolar disorder and depression. And children with ADHD often have symptoms of other developmental disorders, such as autism. In fact, a 2011 study detected genetic changes that were common to both ADHD and autism.

Because these psychiatric disorders share symptoms, researchers have not always agreed on how to classify them. The new findings might one day help researchers reclassify psychiatric conditions in a way that incorporates genetics, the researchers said.

The new study will be published online Feb. 28 in the journal The Lancet.

To find the shared genetic markers, Dr. Jordan Smoller from Massachusetts General Hospital in Boston and colleagues analyzed information from 33,332 people who each had one of the psychiatric disorders and 27,888 people who did not.

Because the study involved only people of European ancestry, it's not clear whether the results apply to other populations as well. In addition, the study only found an association between the four genetic markers and these conditions, but cannot prove that the markers play a role in causing the disorders. (That is, just because a person has one of these markers, does not mean that he or she will develop a psychiatric disorder.)

The findings confirm what many researchers already thought — that genetic risk factors aren't necessarily specific to one disease, said Dr. Joachim Hallmayer, an associate professor of psychiatry at Stanford University who was not involved with the study.

Some psychiatric disorders already share some treatments, such as medications aimed at easing symptoms. But genetic studies like these may reveal new targets for drugs "that go more to the root of the condition and prevent the condition," Hallmayer said.

However, more research is needed to determine how these genetic markers cause problems that might lead to disease, Hallmayer said. Future studies might investigate whether environmental factors could interact with these genetic markers to trigger disease, he said.

Ellen Quillen, a geneticist at Texas Biomedical Research Institute in San Antonio, said genetic studies that analyze several disorders at the same time are important to identify the genetic risk factors these various disorders share. Previously, evidence has shown that multiple disorders run in the same family.

"I hope these results will encourage more researchers to look for shared variants underlying clinically distinct disorders, as well as focusing on particular shared symptoms," Quillen said.

Rachael Rettner

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.